After looking around a bit on here, I realized it's a common issue. Many people suggested that it's lice, mites, molting, other chickens (we do have a rooster), or the hen herself, but I'm really not sure what the problem is here:
Pretty uncommon for a 10 month old(Pullet) chicken to molt.How much space do you have for chickens Ft by Ft?
One of mine had an unexplained bare bottom. She was bare for months and months. I tried everything - thinking it was a pecking issue with the others, lice, mites, etc. The rest of my girls had fuzzy bottoms - it was just her with the bare one. As it turns out, over time I realized she was an internal layer. I could only guess that being full of fluid and egg material, either her feathers didn't grow there or she was plucking them out because she was uncomfortable. But since yours is still laying, that's a good thing! If she happens to stop though, it's something to keep in mind. Hope her feathers grow back soon!